Under standing USB charging.

27 Jan 2008
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Llanfair Caereinion, Nr Welshpool
United Kingdom
I have a rear push bike light which is charged from USB, not really what I wanted, but only one I could find which bolts on to rear bike rack. Put it on charge down stairs and green light comes on and stays on, and after over night charge, switches off after 20 minutes of use, but put on charge upstairs the charge monitor shows it taking 0.20 amps and after a few hours green light goes off, and it will run for about 6 hours.

However same lead and charger socket both up and down stairs will charge phone and tablet without a problem.

Once it is flat again I will move monitor down stairs and see what charge rate is, however puzzling why leads and charging sockets which can charge phone or tablet at around 0.9 - 1.2 amp fails to charge back light at 0.2 amps.

I had problems before, which is why I bought the volt/amp meter that plugs into a USB outlet, in the main I found leads from Pound World was the problem, they worked, but very slowly, and over time I have been getting rid of these leads. However why it should work with one item which draws a high charging current, and not with another which in comparison draws a very low current, I can't understand.
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when you say socket i assume you mean the same type off sockets up and down rather than a long 5v circuit loosing volts as it goes ??

the rate something charges will be based on what its offered and what its circuitry is set up for and wants to take
I have around the house 6 double 13A sockets with USB charging, they free up the 13A sockets, I don't have loads of USB chargers filling up sockets. Some are in use 24/7, one in this room runs the hub controlling the central heating, however that uses just 0.02A.

The ammeter is handy, I can see when a lead is faulty and when something nearly fully charged, so the light in question uses around 0.22 amp when first plugged in, but if already charged then just 0.06 amp, so even without a battery icon, or charge warning lamp, I can see when nearly fully charged. However I only have one meter, so can't say what charge rate is when charged down stairs until used again and meter moved. However down stairs green charge light never goes off, upstairs it goes off completely and before that point reduces in light output. It never seems to dim or go out when charged down stairs.

Clearly some hand shaking goes on, the socket has a max output I think 2.1 amp, the lead also has a max, not a clue what, and the device being charged has a max. How the USB sockets, leads and devices stop overload I don't know, however whole idea of rear light is to ensure car drivers see me, unlike the front lamp, if it fails I do not know it has failed, there is a second red flashing light on my helmet, but very small compared with light on rear rack, rear light on my bike is same brightness as my wife's and she has a 36 volt 400 watt/hour battery powering hers, it also powers drive motor, however designed so it switches off drive motor on hour before lights fail. Mine had only been on 15 minutes when I found no rear lamps, not a clue if lasted 1 or 14 minutes, can't see it when riding.

But today it was on at least 1 hour with no signs of dimming, quite clearly it had not been charged fully when it failed.
if these are individual 13amp sockets with usb then they are completely isolated and disconnected so no connection what so ever so need to be treated as such if charging seems slow then they probably wont be compatable as in input power requirement not matched to output off the power source
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May be I have it wrong? USB = Universal serial bus, I thought the universal means they are all the same? OK there is USB 1, 2 and 3 but all my sockets are USB 2.0 so should all be the same.
Universal serial bus,

More accurately Universal Serial data Bus with additional conductors for supplying power to data devices using the serial data for processing the data.

Using the power for other purposes such as charging items was never part of the original specification.

Many devices ( try to ) exchange data about their identities with the power supply unit before starting to charge.

This was to try and prevent expensive items being damaged by charging from non approved sources of USB power.

EDIT added text in red
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I realise the USB was not originally designed as a charging point, however my USB charging outlets built into 13A sockets clearly are designed only for charging, some leads are not designed for high current charging and in the past this was a problem, the volt/ammeter plug in device was bought so I could separate high power and low power leads, however with a socket and lead which will charge tablet at over 1 amp, I would have thought a small rear light for a bike would have no problem. Unplug and plug back in both outlets tried in it charges at 0.06 amp dropping quickly down to zero, now being fully charged, and the green charge light comes on then fades off. Discharged it does not draw high amps around 0.20 amps, but until discharged again can't test it.
But over night it clearly did not charge from down stairs socket, the green light goes out after around 5 hours upstairs, but was still lit after 10 hours or more down stairs, and when used went flat on me.

So the question is why it didn't charge? And more to point what do I need to do to ensure it does charge over night? The lead is called a Micro USB, they are used with most of what I have, some old stuff has Mini USB and very new has type C which go in either way up, those we have had problems with, when they charge OK one way up but not other way up, also we have some very old USB B leads on printers etc. Plus some specials, my Pentax camera has a special for example.

Looking at size of Micro USB over 1 amp is a lot of power, but they do seem to handle it, and as said the Samsung phone and tablet use far more power to charge with same leads.
Are the down stairs sockets the same as the up stairs sockets ?

I suspect the data lines may be biased with a voltage from a high impedance source and this bias affects whether the ( slightly intelligent ) charging circuit in the item will operate.
Google the TPS2511 device from Texas Instruments.

From the data sheet

The TPS2511 device is a USB-dedicated charging port (DCP) controller and current-limiting power switch. An auto-detect feature monitors USB data line voltage, and automatically provides the correct electrical signatures on the data lines to charge compliant devices among the following dedicated charging schemes:

1. Divider DCP, required to apply 2.7 V and 2 V on the D+ and D– lines respectively or 2 V and 2.7 V on the D+ and D– lines respectively
2. BC1.2 DCP, required to short the D+ line to the D– line
3. 1.2 V on both D+ and D– lines
Change the downstairs socket. It is clearly faulty (or at least its USB outlet is).

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